This invention relates to the technology of efficiently surface-mounting electronic components such as various kinds of semiconductor devices and elements onto a target surface, for example, of a printed circuit board.
As is well known, the so-called solder reflow method is usually used for surface-mounting a plurality of semiconductor devices and elements to a printed circuit board. According to this method, after a solder paste is applied to the surface of terminal-forming areas prepared on the printed circuit board and the semiconductor devices and elements are placed thereon, the circuit board is placed inside a heating oven to cause the solder paste to reflow. This method is convenient because the soldering of a plurality of semiconductor devices can be carried out at once together by heating and melting the solder paste applied at various positions on the printed circuit board. This prior art method, however, has several disadvantages.
Firstly, the solder paste must be applied to the surface of the terminal-forming areas with a high degree of accuracy by using a printing machine. If the solder paste spills over too much from the target surface, there may result an incomplete connection of conductors. In summary, the application of solder paste is a very troublesome work, and this problem is particularly serious where solder paste must be applied at a large number of places. Secondly, this method requires that the printed circuit board be kept inside the heating oven until its temperature reaches the melting temperature of the solder. Thus, electronic components with poor heat resistance cannot be preliminarily mounted to the circuit board. In other words, the order of mounting various components onto the circuit board must be carefully planned such that components with poor heat resistance will not be subjected to a long heating process inside a heating oven.
It is therefore an object of this invention in view of the above to provide a more convenient and efficient method of surface-mounting electronic components to a target surface of a printed circuit board.
A method embodying the invention, by which the above and other objects can be accomplished, may be characterized as comprising the steps of forming an anisotropic conductive layer on the target surface, placing the electronic components on this layer such that their conductive connecting members are individually above the terminal parts in specified terminal-forming areas of the surface, and pressing these electronic components to the anisotropic conductive layer such that corresponding pairs of these conductive connecting members and the terminal parts become mutually attached and electrically conductive through this anisotropic conductive layer while the remaining portions of the layer remain electrically insulative. Such an anisotropic conductive layer may be formed either by attaching a single sheet of an anisotropic conductive film of a thermosetting synthetic resin having conductive particles dispersed therein or by applying an anisotropic conductive adhesive similarly having conductive particles dispersed inside a thermosetting resin material. This layer is preferably heated as the electronic components are pressed against it so as to become advantageously hardened.
The method of this invention is advantageous because the surface-mounting of electronic components to a target surface can be carried out securely and dependably. The anisotropic conductive layer can be melted and softened by heating as a plurality of electronic components are pressed against it. As the heating temperature is increased, the softened anisotropic conductive layer can be hardened more quickly by the thermosetting characteristic of the material than if it were left to naturally become hardened. This invention therefore relates also to surface-mounted structures produced by a method of this invention as described above.